1. Field of the Invention
The present invention relates to an optical head including a plurality of light sources having different wavelengths and adapted to optically record or reproduce information in or from information recording media such as a plurality of kinds of optical discs, an optical disc device including such the optical head, and a computer, an optical disc player and an optical disc recorder including such the optical disc device.
2. Description of the Background Art
In recent years, Blu-ray discs (hereinafter, BDs) which are high-density and large capacity optical information recording media (hereinafter, also referred to as “optical discs”) having the same size as CDs (Compact Discs) and DVDs (Digital Versatile Discs) have been put to practical use as blue-violet semiconductor lasers have been put to practical use. This BD is an optical disc whose protective substrate is about 0.1 mm in thickness and in or from which information is recorded or reproduced using a blue-violet semiconductor laser light source having a wavelength of about 400 nm and an objective lens whose numerical aperture (NA) is increased to 0.85.
HD DVDs whose protective substrate is about 0.6 mm in thickness and in or from which information is recorded or reproduced similarly using a blue-violet semiconductor laser light source having a wavelength of about 400 nm and an objective lens whose numerical aperture is 0.65 have been also put to practical use. These optical discs in or from which information is recorded or reproduced using blue-violet light sources are collectively called high-density optical discs.
Accordingly, there has been proposed an optical head compatible with optical discs whose protective substrates differ in thickness and capable of recording or reproducing information in or from information recording surfaces of these optical discs by focusing laser lights having different wavelengths using one objective lens.
A construction example of such an optical head is shown in FIG. 38. In FIG. 38, identified by 101 is a light source for emitting a blue-violet laser beam, by 102 a beam splitter, by 103 a relay lens, by 104 a dichroic prism, by 105 a collimator lens, by 106 an objective lens, by 107 a light receiving element, by 111 a light emitting and receiving element for emitting and receiving a red laser beam and an infrared laser beam. These parts construct an optical head 130. Further, identified by 60 is a BD which is an optical disc whose protective substrate is 0.1 mm in thickness.
An operation of the optical head 130 to record or reproduce information in or from the BD 60 is described. A blue-violet laser beam emitted from the light source 101 is reflected by the beam splitter 102 and passes through the relay lens 103 to be converted into a divergent beam having a larger divergence angle. The converted divergent beam is converted into a substantially parallel beam by the collimator lens 105 after being reflected by the dichroic prism 104, and is focused as a light spot on an information recording surface of the BD 60 through the protective substrate by the objective lens 106. The laser beam reflected by the information recording surface of the BD 60 passes through the objective lens 106 and the collimator lens 105 again to be reflected by the dichroic prism 104, and passes through the beam splitter 102 to be introduced to the light receiving element 107 after passing through the relay lens 103.
Next, an operation of the optical head 130 in the case of recording or reproduction in or from a DVD 70 which is an optical disc whose protective substrate is 0.6 mm in thickness (or a CD which is an optical disc whose protective substrate is 1.2 mm in thickness) is described with reference to FIG. 39. A red laser beam having a wavelength of 655 nm (or infrared laser beam having a wavelength of 785 nm) emitted from the light emitting and receiving element 111 passes through the dichroic prism 104, is converted into a substantially parallel beam by the collimator lens 105, and is focused as a light spot on an information recording surface of the DVD 70 (or CD 80) through the protective substrate by the objective lens 106. The laser beam reflected by the information recording surface of the DVD 70 (or CD 80) passes through the objective lens 106, the collimator lens 105 and the dichroic prism 104 again to be returned to the light emitting and receiving element 111.
Here, the objective lens 106 has a diffraction structure for focusing a blue-violet laser beam for recording or reproduction in or from the BD 60, a red laser beam for recording or reproduction in or from the DVD 70 and an infrared laser beam for recording or reproduction in or from the CD 80 as minute light spots utilizing wavelength differences.
Accordingly, by using such an optical head 130, information can be recorded or reproduced by focusing laser beams having different wavelengths on different kinds of optical discs by means of one objective lens 106.
Japanese Unexamined Patent Publication No. H10-269607 discloses the use of a wedge-shaped prism to simplify the construction of an optical system in an optical head using a plurality of light sources having different wavelengths. In Japanese Unexamined Patent Publication No. H10-269607 is proposed to arrange an incident surface and an emergent surface of a flat prism at an angle to each other in order to avoid an astigmatism when a convergent beam having a wavelength of 780 nm passes through the prism.
Another prior art is disclosed in Japanese Patent No. 3304053. The schematic construction of an optical head 131 shown in Japanese Patent No. 3304053 is shown in FIG. 40. In FIG. 40, identified by 112 is a first light emitting and receiving element for emitting and receiving a red laser beam (wavelength of 650 nm), by 113 a second light emitting and receiving element for emitting and receiving an infrared laser beam (wavelength of 780 nm), by 114 a wedge-shaped prism, by 115 a collimator lens and by 116 an objective lens.
Here, the objective lens 116 can focus a red laser beam and an infrared laser beam as minute light spots respectively on the DVD 70, which is an optical disc whose protective substrate is 0.6 mm in thickness, and on the CD 80, which is an optical disc whose protective substrate is 1.2 mm in thickness.
In Japanese Patent No. 3304053, an angle (apex angle α) between the incident surface and the emergent surface of the wedge-shaped prism 114, incident angles θ1, θ2 of the red laser beam and the infrared laser beam on the wedge-shaped prism 114 and the like are specified as below (see FIG. 41).
Apex angle α of the wedge-shaped prism 114=2.05°
Thickness T of a middle part of the wedge-shaped prism 114=1.5 mm.
Incident angle θ1 of the infrared laser beam=42.85°
Incident angle θ2 of the red laser beam=39°
Numerical aperture (NA) of the objective lens 116=0.45
At this time, a wavefront aberration when the infrared laser beam having passed through the wedge-shaped prism 114 is gathered by the objective lens 116 is 15 mλrms, and a light spot can be satisfactorily focused.
In the above conventional optical head, the beam that passes through the wedge-shaped prism 114 is the infrared laser beam having a smaller NA and used for CDs, and the laser beam having a larger NA and used for DVDs is reflected by the wedge-shaped prism 114 to be incident on the collimator lens 115.
On the other hand, in the case of constructing a three wavelength compatible optical head for recording or reproduction in or from high-density optical discs such as BDs, and DVDs and CDs, a blue-violet laser beam used for high-density optical discs is preferably reflected by a prism and both a red laser beam used for DVDs and an infrared laser beam used for CDs are emitted from the same light source and passes through the prism as shown in FIGS. 38 and 39. This is because a light source capable of emitting lights of two wavelengths are generally used and advantageous costwise.
However, in the case of using a wedge-shaped prism in a three wavelength compatible optical head for recording or reproduction in or from high-density optical discs such as BDs, and DVDs and CDs, a blue-violet laser beam for recording or reproduction in or from a BD required to have a more strict wavefront aberration characteristic can be reflected by the wedge-shaped prism, thereby substantially avoiding aberration. However, in the case of an optical system for DVDs having a large NA, even if the wedge-shaped prism is designed to minimize only the astigmatism, coma aberration and other aberrations remain to affect a recording or reproduction performance.
This coma aberration and other aberrations are not considered in Japanese Patent No. 3304053. If the disclosed content of Japanese Patent No. 3304053 is used as it is, the coma aberration and other aberrations remain in optical systems for DVDs having a large NA. Therefore, the wedge-shaped prism cannot be used in the three wavelength compatible optical head for recording or reproducing in or from high-density optical discs such as BDs, DVDs and CDs.